Seasonal And Interannual Variability In Wind-driven Upwelling Along The Zhejiang Coast And The California Coast

There are many upwelling regions in the global oceans. There are different generation mechanisms of coastal upwelling at the east and west borders of the oceans, but most of them are produced by coastal wind, as a result of Ekman transport. Upwelling brings deeper water up to the surface or subsurface layer and cause sea water to readjust its physical and chemical properties and biological distribution. Because coastal upwelling can carry nutrients from the deeper layer to the upper layer, it promotes the growth of primary producers such as phytoplankton. As a result, upwelling can indirectly affect the growth and population of fish in coastal regions. Therefore, the study of upwelling has become more and more attention. This thesis focuses on upwelling research, and the main points are the follows:Firstly, Coastal Upwelling Index (CUI) proposed by Bakun to quantify the upwelling intensity is improved, the main improvement is to use the actual wind speed as the initial data and consider the tilt angle of the local coastline. And we turn the offshore surface water volume into the vertical upwelling speed and define this speed as the new CUI. The new CUI can be more intuitive and accurate to express the upwelling intensity. In addition, reducing the interval between the stations is more beneficial to study the spatial variation of upwelling along the whole coast.Secondly, the new CUI expresses the upwelling intensity, which is applied to the Zhejiang coast and the California coast, respectively. The high correlations of the new CUI with the chlorophyll concentration (Chic) at both seasonal and interannual scales demonstrate that it is more suitable for the study of the two coastal upwelling than old CUI. A high eddy kinetic energy (EKE) band is coincidently located at the upwelling front along two different coasts, which implies that the mixing caused by frontal instabilities could further modulate the Ch1c.Finally, the new CUI is used to study the seasonal.and interannual variations of the two coastal upwelling and their spatial distribution characteristics. The Zhejiang coast is strongly influenced by the eastern Asian monsoon. When the southerly monsoon dominates in summer, the coast seasonal wind-driven upwelling is produced. The spatial variation along the Zhejiang coast shows the meridional CUI gradient is monotonically positive northward in summer. The lagged positive correlation between the El Nino Southern Oscillation (ENSO) and CUI, can interpret its long-term variation of upwelling in the Zhejiang coast, with the negative correlation between the Pacific Decadal Oscillation (PDO) and CUI. The California coast is always controlled by the northerly wind from March to November or December in the year. Both the Chlc and the CUI rises to a maximum in the summer. The seasonal and interannual variations of the Chlc are related to the upwelling in the California coast.